In the step of machining steel sheets under pressure to produce automobile bodies or the like, high-viscosity press oil is scarcely used, since the steel sheets being machined in the step require efficient working, degreasing and chemical treatment. In general, therefore, the rust preventing oil coated on the steel sheets is directly used as the press oil. However, such is problematic in that the machining of the parts with complicated shapes often result in press cracking and other troubles. In order to solve the problems, expensive materials having a high mean-r value (that is, having good press formability) are employed at present in the art. Given the situation, therefore, it is desired to develop inexpensive materials with good press formability which can be machined under pressure to give any parts with complicated shapes.
On the other hand, in the field of electric appliances, steel sheets are machined in the presence of press oil, and the machined products are degreased with a freon-type solvent before use. However, the freon-type solvent is a chemical that destroys the ozone layer around the earth, and the working environment where press oil is used is not good for factory workers. For these reasons, the recent tendency is toward the development of organic composite coated steel sheets capable of being press-formed in the absence of oil.
Under the situation mentioned above, various organic composite coated steel sheets such as those mentioned below have heretofore been proposed.
(1) Japanese Patent Application laid open (KOKAI) under No. Sho 64-8033 discloses an organic composite coated steel sheet, which comprises a Zn or Zn alloy plated steel sheet having on its surface a first chromate film and a second organic film of an amine-modified epoxy resin containing silica or a chromate, in that order.
(2) Japanese Patent Application laid open (KOKAI) under No. Hei 3-284942 discloses an organic resin-coated, galvannealed steel sheet in which the organic film contains silica or a fine powder of a fluorine resin to thereby improve the anti-powdering property and the anti-flaking property of the steel sheet.
(3) Japanese Patent Application laid open (KOKAI) under No. Hei 6-173037 discloses an organic composite coated steel sheet in which the organic film contains an ether-ester type urethane resin, an epoxy resin, silica and a polyolefin wax.
(4) Japanese Patent Application laid open (KOKAI) under No. Hei 5-237449 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet having on its surface a first chromate film and a second organic film in that order, and in which the second organic film comprises a resin having hydroxyl and/or carboxyl groups, large particles of a polyolefin wax (having a mean particle size of from 3.0 to 5.0 .mu.m), small particles of a polyolefin wax (having a mean particle size of 1.0 .mu.m or smaller) and silica.
(5) Japanese Patent Application laid open (KOKAI) under No. Hei 5-65667 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet having on its surface a first chromate film and a second organic film in that order, and in which the second organic film comprises an aqueous resin, silica and two different polyolefin waxes with a molecular weight of from 1000 to 4000, one polyolefin wax having a softening point of 120.degree. C. or lower while the other having a softening point of higher than 120.degree..
(6) Japanese Patent Application laid open (KOKAI) under No. Hei 6-59455 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet having on its surface a first chromate film and a second organic film in that order, and in which the second organic film comprises an organic resin, silica and a solid crystalline lubricant (of which 10% or more is a high-melting-point, large-particulate lubricant comprising large particles having particle sizes of 20 .mu.m or larger and a high melting point of 120.degree. C. or higher).
(7) Japanese Patent Application laid open (KOKAI) under No. Hei 5-255587 discloses an organic composite coated steel sheet, comprising a cold-rolled steel sheet or a zinc or zinc alloy plated steel sheet of which the surface is treated through some surface treatment and coated with an organic film that comprises a water-dispersible polyurethane resin, silica, and a polyolefin wax and/or a fine powder of a fluorine resin.
(8) Japanese Patent Application laid open (KOKAI) under No. Hei 6-57440 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet or an aluminium or aluminium alloy plated steel sheet having on its surface a first chromate layer and a second layer of an organic film in that order, and in which the organic film comprises a carboxyl-modified epoxy resin or a polyvinyl butyral resin, silica, a polyethylene wax and a fine powder of a tetrafluoroethylene resin.
(9) Japanese Patent Application laid open (KOKAI) under No. Hei 5-65666 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet having on its surface a first chromate layer and a second layer of an organic film in that order, and in which the organic film comprises an aqueous resin, silica, a dispersion of a polyolefin wax with polar groups having a molecular weight of from 1000 to 4000 and having a softening point of 150.degree. C. or lower (particle size: 3.0 .mu.m or smaller) and a Teflon dispersion (particle size: 3.0 .mu.m or smaller).
(10) Japanese Patent Application laid open (KOKAI) under No. Hei 2-43040 discloses an organic composite coated steel sheet, which comprises a zinc or zinc alloy plated steel sheet having on its surface a first chromate layer and a second layer of an organic film in that order, and in which the organic film comprises a carboxyl-modified epoxy resin or a polyvinyl butyral resin, silica, a polyolefin wax having a melting point of 70.degree. C. or higher and a fine powder of a fluorine resin (particle size: 1.0 to 7.0 .mu.m).
(11) Japanese Patent Application laid open (KOKAI) under No. Hei 3-2257 discloses an organic composite coated steel sheet, which comprises a cold-rolled steel sheet or a zinc or zinc alloy plated steel sheet having on its surface a first phosphate or chromate film and a second organic film in that order, and in which the second organic film comprises an epoxy resin, silica, a polyolefin wax having a melting point of 110.degree. C. or higher (mean particle size: 2.0 to 5.0 .mu.m, specific gravity: 0.94 to 0.98) and a fine powder of a fluorine resin.
However, the above-mentioned, conventional, organic composite coated steel sheets are all problematic, for example, in the following points.
In the organic composite coated steel sheet of the above-mentioned (1), the organic film is effective for preventing the adhesion of the plated base sheet to a press mold in some degree. In this, however, since the organic film does not specifically contain any lubricant, the lubricative property of the organic film coated sheet is not superior to that of the plated base sheet. Thus, since there is no improvement in the lubricative property of the sheet (1), the press formability thereof is not improved to such a degree that the sheet (1) may be graded up to a one-rank higher one.
For the organic composite coated steel sheet of the above-mentioned (2), the anti-powdering property and the anti-flaking property of the galvannealed steel sheet base thereof may be improved in some degree. However, the ability of the sheet (2) to adhere to a film as formed thereon through cationic electrodeposition is somewhat poor. In addition, since only fine powder of an ordinary fluorine resin is in the organic film as the lubricant, the organic film could not bring about the improvement in the press formability of the sheet (2) to such a degree that the sheet (2) may be graded up to a one-rank higher one.
In the organic composite coated steel sheets of the above-mentioned (3), (4) and (5), one or more different polyolefin waxes are in the organic film as the lubricant. Therefore, the lubricative property of these sheets is good in oil-free forming. However, if these sheets are formed into automobile bodies or the like, while being coated with a rust preventive oil or with a wash oil, such polyolefin waxes could not exhibit by themselves satisfactory lubrication as in oil-free forming. In such forming with oil, the organic films of these sheets could not bring about the improvement in the press formability of these sheets to such a degree that the sheets may be graded up to one-rank higher ones.
The lubricative property of the organic composite coated steel sheets of the above-mentioned (6) and (8) is good in oil-free press forming. In these sheets, however, the lubricant in the organic film comprises an ordinary polyolefin wax or fine powder of an ordinary fluorine resin. Therefore, if these sheets are formed into automobile bodies or the like, while being coated with a rust preventive oil or with a wash oil, their lubrication is not good as in oil-free forming. In such forming with oil, the organic films of these sheets could not bring about the improvement in the press formability of these sheets to such a degree that the sheets may be graded up to one-rank higher ones. In addition, the ordinary, fine-powdery fluorine resin used in these sheets as the lubricant is defective in that its dispersion stability in coating compositions is poor.
For the organic composite coated steel sheets of the above-mentioned (7) and (9), their press formability in oil-free conditions is improved. However, since the organic films of these sheets comprise a water-soluble or water-dispersible resin, they are defective in that they easily absorb water in a corrosive environment or in a wet environment, resulting in poor corrosion resistance of the sheets and poor paint adhesion to the sheets. In addition, since the organic films of these sheets comprises an ordinary polyolefin wax or fine powder of an ordinary fluorine resin as the lubricant, the lubricative property of the sheets is poor in forming with oil, being different from that in oil-free forming. Thus, the organic films of these sheets could not bring about the improvement in the press formability of these sheets to such a degree that the sheets may be graded up to one-rank higher ones.
The lubricative property of the organic composite coated steel sheets of the above-mentioned (10) and (11) is good in oil-free press forming. In these sheets, however, since the lubricant in the organic film comprises fine powder of an ordinary fluorine resin, the lubricative property of the sheets is poor in forming with oil, being different from that in oil-free forming. Thus, the organic films of these sheets could not bring about the improvement in the press formability of these sheets to such a degree that the sheets may be graded up to one-rank higher ones. In addition, the ordinary, fine-powdery fluorine resin used in these sheets as the lubricant is defective in that its dispersion stability in coating compositions is poor.
Attention has lately come to be drawn to corrosion resistance in a corrosive environment in which iron rust is present (hereinafter referred to as "corrosion resistance in a rust-contaminated environment") [CAMP-ISIJ, Vol. 5 (1992), p. 1693]. It has been pointed out that the exposure of an organic composite coated steel sheet to such a rust-contaminated environment results in iron rust adhering to the surface of its organic resin film, thereby causing a great reduction in its intrinsic, excellent corrosion resistance to such an extent that the thus-exposed sheet is no longer appreciably superior in corrosion resistance to any ordinary zinc or zinc alloy plated steel sheet having no organic resin film thereon. Therefore, it is desired to make organic composite coated steel sheets have further improved corrosion resistance even in a rust-contaminated environment. However, the level of the corrosion resistance of the conventional organic composite coated steel sheets, such as those of the above-mentioned (1) to (11), in a rust-contaminated environment is not still satisfactory.
GALVATECH '92 (p. 372) states that there was obtained an organic composite coated steel sheet having a lower corrosion resistance in a rust-contaminated environment when the crosslinking density of its organic resin film was lowered by reducing the amount of the crosslinking agent added to the organic resin constituting the film. It however, fails to describe any specific means for improving the corrosion resistance of the organic composite coated steel sheet in a rust-contaminated environment, though the above statement may suggest that the increase in the crosslinking density of the organic resin film may result in the improvement in the corrosion resistance of the sheet in a rust-contaminated environment.
Having taken these problems in the prior art into consideration, we, the present inventors have made the present invention, of which the object is to realize the improvement in the press formability of an organic composite coated steel sheet to that of a one-rank higher one (by about 0.2 in terms of the mean-r value of the sheet) by improving the lubricative property of the organic film to be formed on the surface of the sheet, while improving the perforation corrosion resistance of the sheet and even the corrosion resistance thereof in a rust-contaminated environment.